Many members of the negative strand RNA viruses cause diseases of the central nervous system (CNS) and are, therefore, important to medical neurology. Infections frequently convert to a persistent state either by the formation of defective interfering (DI) particles, deletion mutants which interfere with the replication of the parental virus or by the accumulation of mutations in the ever changing genome, both affecting the amount of virus released and its cytopathogenicity. Little is known about the regulation of gene expression and replication of these viruses in host cells. All of these processes involve the polymerase and its specific interactions with the nucleocapsid template. The molecular events of transcription and replication of the viral genome are subject of this research project. Towards these ends, we have cloned and sequenced the VSV L gene (6,400 bases) which codes for the multifunctional RNA dependent RNA polymerase (L). The sequence analysis revealed direct evidence for the high mutability of VSV, suggesting that the polymerase itself has a mutator function which may contribute to the establishment and maintenance of persistent infections. In order to study the multiple viral essential functions of the polymerase, we have assembled the complete L gene from partial cDNA clones and have expressed this gene in eukaryotic cells. This represents the first successful functional expression of a recombinant polymerase gene of a negative strand virus. This system will now allow for the first time to dissect the functions of the protein as well as their structural organization within this single gene. We anticipate that the study of these functions will reveal characteristic viral specific mechanisms which may be exploited in the treatment or prevention of viral infections in the CNS.